Continuous digital ratemeter

ABSTRACT

A continuous ratemeter having a digital output is described. The ratemeter has relatively long integration periods while producing a substantially continuous output. The ratemeter develops a delayed replica of the input pulse train and can indicate differential as well as integral count rates.

United States Patent Rudnick et al.

CGNTINUOUS DIGITAL RATEMETER Inventors: Stanley J. Rudnick, Naperville; Paul L.

Michaud, Lockport; Karl G. Porges, Evanston, all of 111.

The United States of America as represented by the United States Atomic Energy Commission May 28, 1970 Assignee:

Filed:

Appl. No.: 41,457

US. Cl ..23s/1s0.3, 324/78 ..G0lr 23/02 Field of Search ..235/150.31, 150.3; 328/165,

[ Feb. 29, 1972 Primary Examiner-Malcolm A. Morrison Assistant Examiner-Edward J. Wise A ttorneyRoland A. Anderson ABSTRACT A continuous ratemeter having a digital output is described. The ratemeter has relatively long integration periods while producing a substantially continuous output. The ratemeter develops a delayed replica of the input pulse train and can indicate differential as well as integral count rates.

6 Claims, 3 Drawing Figures DIG/T194 0U TPUT DZSPL l7) l5 CONTINUOUS DIGITAL RATEMETER CONTRACTUAL ORIGIN OF THE INVENTION The invention described herein was made in the course of, or under, a contract with the UNITED STATES ATOMIC ENERGY COMMISSION.

BACKGROUND OF THE INVENTION Ratemeters are widely used in reactor instrumentation, alarm systems and other nuclear applications. While a large number of stable analog ratemeter designs are available, the input to a ratemeter is digital and an output which is also digital is often required, for example, when the ratemeter is used in conjunction with an alarm level discriminator. Furthermore, digital computers are increasingly employed to monitor, diagnose and correct power levels of reactors and feed rates of processing plants. With existing analog ratemeters it is necessary to develop an analog function from the digital output of a count channel and then develop a digital signal from the analog output for alarm system or computer input.

A number of digital ratemeters of the cyclic count and dump type have been described and are commercially available. These cyclic count and dump circuits produce outputs only at the end of each count interval and respond to step inputs in a manner determined not only by the choice of counting periods but also by the particular phase of the count cycle at the instant the step occurs.

It is therefore an object of this invention to provide a continuous digital ratemeter having relatively simple logic.

Another object of this invention is to provide a continuous digital ratemeter which produces a delayed replica of the input pulse train.

Another object of this invention is to provide a continuous digital ratemeter which is capable of differential counting.

Another object of this invention is to provide a continuous digital ratemeter having an output which is always present and in which the response to a step change in input begins immediately.

SUMMARY OF THE INVENTION In practicing this invention, a continuous digital ratemeter is provided for receiving random input pulses and for developing an output representative of the input pulse rate. The ratemeter includes a clock for developing a clock signal having a frequency f. A derandomizer receives the random input pulses and acts to develop only a single pulse output during a period l/f in response to at least one random input pulse being received during the period l/f. A shift register is coupled to the clock means and has an input coupled to the derandomizer for receiving the pulse outputs therefrom. An add-subtract scaler has an add input mode coupled to the derandomizer and a subtract input mode coupled to the shift register. Each input pulse from the derandomizer adds one to the count in the add-subtract scaler. Each pulse output from the shift register subtracts one from the add-subtract scaler. Thus the number of counts in the sealer is always exactly equal to the number of pulses in the shift register.

The derandomizer may be used to reduce errors which occur when more than one data input pulse is received during a period l/f. The ratemeter may also be constructed to provide a differential count rate to detect sudden changes in the count rate while excluding longtime changes. The information stored in the shaft register is a delayed replica of the input pulse train. This information may be read out to show how a particular count in the add-subtract scaler was produced.

BRIEF DESCRIPTION OF THE DRAWINGS The invention is illustrated in the drawings, of which: FIG. I is a block diagram of the continuous digital ratemeter of this invention.

LII

FIG. 2 is a block diagram of the continuous digital ratemeter of FIG. 1, showing a derandomizer which reduces errors caused by multiple input pulses during the count period.

FIG. 3 is a block diagram of a continuous digital ratemeter which develops a differential count.

DETAILED DESCRIPTION OF THE INVENTION Referring to FIG. I, there is shown a block diagram of the continuous digital ratemeter of this invention. The input data is applied to the input of the derandomizer 10. Clock II is coupled to derandomizer 10 to control the operation of the derandomizer, The clock signals from clock 11 have a frequency f with a period l/f. Derandomizer l0 acts to produce no more than one output pulse during the period l/f, provided at least one input pulse is received during this period. This prevents additional input pulses during the period 1/)" from being counted improperly to develop count errors. Since the input pulses are random, it is possible for two or more pulses to occur during the period l/f. Since the second pulse would not be counted, an error would result. The circuit in FIG. 2 acts to reduce this error and its operation will be explained in a subsequent portion of this specification.

The output pulses from derandomizer 10 are coupled to shift register 13 and the add-subtract scaler 14 in the add mode. Thus, every pulse developed by derandomizer I0 acts to add one to the count in add-subtract sealer 14. Clock 11 steps the pulses through shift register 13 and the output pulses from shift register 13 are coupled to add-subtract scaler 14 in the subtract mode. As a result, the number of counts in the add-subtract scaler 14 is always exactly equal to the number of pulses in the shift register. This number equals the number of input pulses in the preceding time interval m/f and has an average value mn/f, where m the number of bits in the shift register, f the clock frequency and n the mean input rate.

In an example of a digital ratemeter of this type, the shift register 13 had a capacity of 20,000 bits and the clock frequency was I MHz. The count in add-subtract scaler 14 can be displayed by display unit 16 or coupled to other devices such as a computer as desired. Since the count in add-subtract scaler 14 is exactly the same as the count in shift register 13, the input data which produces a particular count in the add-subtract scaler 14 can be read out and analyzed as desired. This is particularly important in a nuclear safety device wherein a count of a predetermined magnitude within the add-subtract scaler 14 would cause the reactor to shut down or make other changes. The input data which cause this action may then be examined without the necessity for providing permanent storage of all input data received. A reset circuit 17 provides resetting pulses to initialize add-subtract scaler l4 and shift register 13.

Referring to FIG. 2, there is shown a derandomizer circuit which reduces the error caused when more than one pulse is received during the period I If. Input data pulses are received by primary buffer (D-type bistable multivibrator) 20 which produces an output upon receipt of the first pulse in the period l/f. With the primary buffer set in the Q state, a Q output is developed from synchronizer 21 at the proper time in the clock cycle. The Q output from synchronizer 21 is coupled to AND-gate 23 to provide an enabling signal for the AND-gate. The Q output of primary buffer 20 enables auxiliary buffer 28. If no further pulses are received during the period l/f, a clock pulse from adjustable clock 26, delayed for a time equal to or greater than the propagation delay through synchronizer 21 by delay 27, acts to develop an output signal from AND-gate 23, resetting primary buffer 20.

If, however, a second pulse is received during the period I If, the pulse sets enabled auxiliary buffer 28. With auxiliary buffer 28 in the set state, the O output, which is coupled to AND-gate 23, is removed, blocking the AND gate. If a third pulse is received during the period l/f, this pulse is discarded and is lost, producing an error. The probability that three or more pulses will be received during the period I /f is very much less than the probability that one or two pulses will be received. Thus, the circuit decreases the error caused by discarding pulses. If required, additional auxiliary buffers of the same design could be used to further reduce the error.

When AND-gate 23 is disabled by the output from auxiliary buffer 28, the clock pulse from adjustable clock 26 will not reset primary buffer 20. Auxiliary buffer 28 is reset by the clock pulse delayed by both delay 27 and delay 29, whose delay must be equal to or greater than the duration of the pulse from delay 27. Thus, during the following period l/f, primary buffer is in the set state, as if it had received a pulse during that period. If a pulse is received during that period, it is registered in auxiliary buffer 28, as previously described. If, however, a second pulse is received during the subsequent period, it is discarded. The probability that two consecutive periods will each have two or more pulses is very low. Therefore, the use of the single auxiliary buffer stage acts to decrease significantly the errors caused by discarding pulses. Additional auxiliary buffer stages can be added to further reduce the errors, if this is required.

The output of synchronizer 21 is coupled to shift register 30 and the add input of add-subtract scaler 31 through AND-gate 33. The output of shift register 30 is coupled to the subtract input of add-subtract scaler 31 through AND-gate 34. AND- gates 33 and 34 are enabled at the desired time by signals from adjustable clock 26. Reset 35 and display 36 are as previously described in FIG. 1. The operation of shift register 30 and add subtract scaler 31 is the same as previously described. The clock rate of clock 26 can be adjusted as desired to change the period 1/ and consequently the overall delay and time constant.

Referring to FIG. 3, there is shown a continuous digital ratemeter having a circuit which provides a differential count rate. In some nuclear applications the count rate itself is not significant, since the count rate gradually increases over a long period of time. An example might be found in the radioactivity of the primary sodium coolant which will increase as the reactor is operated. However, if a fuel rupture occurs, the radioactivity will increase suddenly and it is important to detect this sudden increase rather than the absolute magnitude of the input data rate. In FIG. 3, the data is applied to derandomizer 40 which acts in the manner previously described. The output of the derandomizer is coupled to the add-subtract scaler 41 to add one to the scaler. The output of derandomizer 40 is also coupled to shift register 43. The output of shift register 43 is coupled through AND-gate 44 to shift register 46 and addsubtract scaler 41. The output from shift register 43 acts to subtract two from the count in the add-subtract scaler 41. The output of shift register 46 is coupled to add-subtract scaler 51 and acts to add one to the count therein. Thus, the count in add-subtract scaler 41 is equal to the difference between the counts in shift register 43 and shift register 46 (count in shift register 43 minus the count in shift register 46). The count in shift register 46 represents the data bits received during the time period T equal to m/f, where m is the number of stages in the shift register and f is the clock frequency of clock 48. The count in shift register 43 represents the number of input data bits received in the period T (equal to m/fl which follows T.. Thus the difference between the counts of shift registers 43 and 46 represents the change in the input data rate in consecutive timer periods and is independent of long-term changes. Reset 49 and display 50 are as previously described.

The embodiments of the invention in which exclusive propetty or privilege is claimed are defined as follows:

1. A continuous digital ratemeter for receiving random input pulses and for developing an output representative of the pulse rate thereof, including in combination, clock means for developing a clock signal having a frequency f, input means for receiving the random input pulses and coupled to said clock means, said input means acting to develop only a single pulse output during a period 1/1" in response to at least one random input pulse being received during said period 1/],

shift register means coupled to said clock means and further avmg an input coupled to said input means for receiving said pulse outputs therefrom and an output, an add-subtract scaler having an add input mode and a substrate input mode, one of said add and subtract input modes being coupled to said input means, the other of said add and subtract input modes being coupled to said shift register means output, the total count in said add-subtract scaler being a measure of the pulse rate of the random input pulses.

2. The continuous digital ratemeter of claim 1 wherein, said add input mode is coupled to said input means and said subtract input mode is coupled to said shift register means output.

3. The continuous digital ratemeter of claim 2 wherein, said clock frequency f is adjustable.

4. The continuous digital ratemeter of claim 3 wherein, said input means includes first buffer means and second buffer means each adapted to receive the random input pulses, said first buffer means having an output coupled to said second bufier means, said first buffer means being responsive to the first random input pulse during said period l/f to develop said single pulse output, said single pulse output acting to enable said second buffer means whereby a second random input pulse during said period l/f is entered into said second buffer means, said second buffer means being responsive to the second random input pulse to develop a disabling signal, control means coupled to said clock means a said second buffer means and said first buffer means, said control means being responsive to said disabling signal to prevent resetting of said first buffer means at the end of said period l/f, whereby the second random input pulse is counted during the time period subsequent to said 1 [1 time period.

5. The continuous digital ratemeter of claim 3 wherein, said shift register means includes a first shift register coupled to said clock means and having an input coupled to said input means for receiving said pulse outputs therefrom and an output, a second shift register coupled to said clock means and having an input coupled to said output of said first shift register and an output, said add-subtract scaler having an add 1 input mode coupled to said input means, an add 1 input mode coupled to said output of said second shift register and a subtract 2 input mode coupled to said output of said first shift register, whereby the count in said add-subtract scaler is equal to the count in said first shift register minus the count in said second shift register.

6. The continuous digital ratemeter of claim 2 wherein, said add input mode of said add-subtract scaler adds 1 to the count therein and said subtract input mode of said add-subtract scaler subtracts I from the count therein. 

1. A continuous digital ratemeter for receiving random input pulses and for developing an output representative of the pulse rate thereof, including in combination, clock means for developing a clock signal having a frequency f, input means for receiving the random input pulses and coupled to said clock means, said input means acting to develop only a single pulse output during a period 1/f in response to at least one random input pulse being received during said period 1/f, shift register means coupled to said clock means and further having an input coupled to said input means for receiving said pulse outputs therefrom and an output, an add-subtract scaler having an add input mode and a substrate input mode, one of said add and subtract input modes being coupled to said input means, the other of said add and subtract input modes being coupled to said shift register means output, the total count in said add-subtract scaler being a measure of the pulse rate of the random input pulses.
 2. The continuous digital ratemeter of claim 1 wherein, said add input mode is coupled to said input means and said subtract input mode is coupled to said shift register means output.
 3. The continuous digital ratemeter of claim 2 wherein, said clock frequency f is adjustable.
 4. The continuous digital ratemeter of claim 3 wherein, said input means includes first buffer means and second buffer means each adapted to receive the random input pulses, said first buffer means having an output coupled to said second buffer means, said first buffer means being responsive to the first random input pulse during said period 1/f to develop said single pulse output, said single pulse output acting to enable said second buffer means whereby a second random input pulse during said period 1/f is entered into said second buffer means, said second buffer means being responsive to the second random input pulse to develop a disabling signal, control means coupled to said clock means a said second buffer means and said first buffer means, said control means being responsive to said disabling signal to prevent resetting of said first buffer means at the end of said period 1/f, whereby the second random input pulse is counted during the time period subsequent to said 1/f time period.
 5. The continuous digital ratemeter of claim 3 wherein, said shift register means includes a first shift register coupled to said clock means and having an input coupled to said input means for receiving said pulse outputs therefrom and an output, a second shift register coupled to said clock means and having an input coupled to said output of said first shift register and an output, said add-subtract scaler having an add 1 input mode coupled to said input means, an add 1 input mode coupled to said output of said second shift register and a subtract 2 input mode coupled to said output of said first shift register, whereby the count in said add-subtract scaler is equal to the count in said first shift register minus the count in said second shift register.
 6. The continuous digital ratemeter of claim 2 wherein, said add input mode of said add-subtract scaler adds 1 to the count therein and said subtract input mode of said add-subtract scaler subtracts 1 from the count therein. 